The human spine is composed of the spinal cord and its protective and supportive coverings, called the vertebral column or backbone.
The spinal cord is a thin tubular extension of the brain, that along with it, makes up the central nervous system. Like the brain, it has a hollow center that is filled with fluid (cerebrospinal fluid). The delicate tissues of the spinal cord are enclosed in and protected by the bony vertebral column. This column is series of stacked bones called vertebrae, discs of a cushioning material between them, and long strands of fibrous ligaments that support the arrangement. "Two concepts critical to understanding the mechanics and pathologic processes affecting the spine are stability and neural compression." (Michael L. Smith, M. Sean Grady. Chapter 41. Neurosurgery in Schwartz's Principles of Surgery, 8th Edition,ISBN: 0-07-141090-2)
Each vertebra is interconnected to the next by an intricate maze of muscles, ligaments and connective tissue with a spongy disc pad between them. Each pair of vertebrae and their supporting structures are particularly responsible for a specific and limited range of movement. Together they create a movable column with the spinal cord sending and receiving the nerve signals that allow for the full ranges of motion that we all enjoy. To make it all work together, a vast array of nervous tissue wraps itself in and among the joint capsules, ligaments and muscles in a web-like fashion constantly transmitting information. These nerves have differing functions; some are afferent (sensory) nerves; some of these monitor how much stretch is being applied to the ligaments or muscles (i.e. mechanoreceptors), others send information about joint position (proprioception) to let the brain know where each joint is and what direction it is moving, and others carry information about temperature and pain. Other spinal nerves are efferent nerves (motor); some of these stimulate tiny muscle groups as necessary to adapt to the constant minor changes in posture and larger groups to maintain proper balance; others control blood flow in parts of the body, and provide growth factors that are important for maintaining tissue and organ function. The spine can perform significant ranges of motion and shock absorption.The mechanoreceptors communicate through the spinal cord to the posture control centers of the cerebellum, and are integrated into the voluntary and involuntary whims or wishes of whatever motion or function the higher centres of the brain decides it wants to perform. The result is a remarkably resilient and mobile structure that allows for upright posture and the ability to perform complicated combinations of movement without conscious planning of every detail of the movements. The strength of the athlete and the agility of the gymnast are testaments to the flexibility and durability that are built into its design.
The spinal cord extends from the medulla oblongata in the brainstem to the conus medullaris near the lumbar level at L1-2, ending in a fibrous extension known as the filum terminale. In an adult, the spinal cord is approximately 18 inches long. The peripheral regions of the cord contain neuronal white matter tracts that contain sensory and motor neurons. In the centre of the cord is a fluid-filled channel called the 'central canal'; this is filled with cerebrospinal fluid, and is continuous with the fourth cerebral ventricle; this is the fluid that is sampled when a lumbar puncture is taken. The three meninges that cover the spinal cord -- the outer dura mater, the arachnoid membrane, and the innermost pia mater; these are continuous with those in the brainstem and cerebral hemispheres, with cerebrospinal fluid found in the subarachnoid space. The cord is stabilized by the connecting denticulate ligaments which extend from the pia mater laterally between the dorsal and ventral roots.
Spinal cord segments
The spinal cord has 30 segments. From each segment, motor nerve roots leave from the ventral side, and sensory nerve roots enter from the dorsal side. The ventral and dorsal roots join to form paired 'spinal nerves', one on each side of the spinal cord.
There are 30 spinal cord segments:
- 7 cervical segments
- 12 thoracic segments
- 5 lumbar segments
- 5 sacral segments
- 1 coccygeal segment
Because the vertebral column grows longer than the spinal cord, spinal cord segments become higher than the corresponding vertebra, especially in the lower spinal cord segments in adults. In a fetus, the vertebral levels originally correspond with the spinal cord segments. In the adult, the cord ends around the L1/L2 vertebral level at the conus medullaris. For example, the segments for the lumbar and sacral regions are found between the vertebral levels of T9 and L2.
The S4 spinal nerve roots arise from the cord around the upper lumbar/lower thoracic vertebral region, and descend downward in the vertebral canal. After they pass the end of the spinal cord, they are considered to be part of the cauda equina, a structure containing nerve roots (the name is Latin for "horse's tail", reflecting its appearance). The roots for S4 leave the spinal cord in the region of the sacrum, a large, triangular bone at the base of the spine, where it is inserted like a wedge between the two hip bones.
There are two regions where the spinal cord enlarges:
- Cervical enlargement - corresponds roughly to the brachial plexus nerves, which innervate the upper limbs. It includes spinal cord segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same (C4 to T1).The brachial plexus is an arrangement of nerve fibres (a plexus) running from the spine (vertebrae C5-T1), through the neck, the axilla (armpit region), and into the arm. The brachial plexus is responsible for cutaneous and muscular innervation of the entire upper limb, with two exceptions: the trapezius muscle innervated by the spinal accessory nerve and an area of skin near the axilla innervated by the intercostobrachialis nerve.
- Lumbosacral enlargement - corresponds to the lumbosacral plexus nerves, which innervate the lower limbs. It comprises the spinal cord segments from L2 to S3, and is found about the vertebral levels of T9 to T12.
Intervertebral discs (or intervertebral fibrocartilage) lie between adjacent vertebrae in the spine. Each disc forms a cartilaginous joint to allow slight movement of the vertebrae, and acts as a ligament to hold the vertebrae together.
Each disc consist of an annulus fibrosus, surrounding the nucleus pulposus, which consists of several layers of fibrocartilage. The strong annular fibers contain the nucleus pulposus and distribute pressure evenly across the disc. The nucleus pulposus contains loose fibers suspended in a mucoprotein gel with a jelly-like consistency. The nucleus acts as a 'shock absorber', absorbing the impact of the body's daily activities and keeping each pair of vertebrae separated. When one develops a prolapsed disc the nucleus pulposis is forced out of the disc and may put pressure on the nerve located near the disc, giving rise to the symptoms of sciatica.
There is a disc between each pair of vertebrae, except for the first cervical segment, the atlas. The atlas is a ring around the roughly cone-shaped extension of the axis (the second cervical segment). The axis acts as a post around which the atlas can rotate, allowing the neck to swivel. There are twenty-three discs in the spine, which are identified by specifying the particular vertebrae they separate. For example, the disc between the fifth and sixth cervical vertabrae is "C5-6".
In the human fetus, the spinal cord extends all the way down to the sacral vertebrae. As a person matures, the rest of the body grows faster than the spinal cord, so that by adulthood, the spinal cord reaches only to the level of the first or second lumbar vertebrae. There is no spinal cord below this level, only the individual spinal nerves that form the cauda equina.
Spinal cord injuries can be caused by falling on the neck or back, or having the spinal cord moved or disrupted in another way. The vertebral bones or intervertebral disks can shatter, causing the spinal cord to be punctured by a sharp fragment of bone. Usually, victims of spinal cord injuries will suffer loss of feeling in certain parts of their body. In milder cases a victim might only suffer loss of hand or foot function. More severe injury may result in paraplegia, tetraplegia, or full body paralysis below the site of injury to the spinal cord.
The two areas of the spinal cord most often injured are the cervical spine (C1-C7) and the lumbar spine (L1-L5). (The prefixes C,T and L refer to the location of a vertebra in the cervical, thoracic, or lumbar region of the spine.)